This invention relates to retinal display devices, and more particularly to optical configurations for retinal display devices and a method and apparatus for defining an exit pupil through which a user views an image.
A retinal display device is an optical device for generating an image upon the retina of an eye. Conventional retinal scanning displays use a coherent light source which is scanned in raster fashion onto the retina. Light is emitted from a light source, collimated through a lens, then passed through a scanning device. The scanning device defines a scanning pattern for the light. In a pre-objective scanning subsystem, the scanned light passes through an objective lens which converges the light to focus an image. Conventionally such light is converged to a flat image plane. The light then diverges beyond the plane. An eyepiece is positioned along the light path beyond the objective lens at some desired focal length. An "exit pupil" occurs shortly beyond the eyepiece in an area where a viewer's eye pupil is to be positioned.
A viewer looks into the eyepiece to view an image. The eyepiece receives light that is being deflected along a raster pattern. Light thus impinges on the viewer's eye pupil at differing angles at different times during the scanning cycle. This range of angles determines the size of the image perceived by the viewer. Modulation of the light during the scanning cycle determines the content of the image.
Typically the exit pupil defined by the display device is less than 2 mm in diameter and often less than 1 mm in diameter. The viewer's eye pupil varies from approximately 2 mm in diameter under bright light to approximately 7 mm in a dark room. Because of the small exit pupil, a first step for a viewer is to adjust eye position to find the exit pupil. The viewer's pupil needs to achieve and maintain alignment with the display device's exit pupil. While in alignment, the light scans directly onto the viewer's retina without any intermediary screens, cathode ray tubes (CRT's) or liquid crystal display devices (LCD's). The result is an image perceived by the viewer.
A shortcoming of the conventional retinal display is the difficulty of maintaining alignment between the exit pupil and the viewer's pupil. If the viewer moves, alignment may be lost. Movement is problematic because a viewer has a tendency to move their eye when intending to view a peripheral portion of the image. Even blinking may cause movement of the eye. As a result, conventional exit pupils are inconvenient for the viewer. In particular a lay consumer using a virtual retinal display would find the alignment requirement difficult to maintain for entertainment or other long term viewing applications. Accordingly there is a need for a retinal display device having an exit pupil defined so as to enable easier viewing of the image.
Other shortcomings of conventional retinal display devices include the display's size and weight. As the retinal display device is to be positioned in the vicinity of a viewer's eye, there is a need to achieve a lightweight compact display device.